The present invention relates to a metallocene catalyst component for use in preparing isotactic polyolefins, especially polypropylenes. The invention further relates to a catalyst system which incorporates the metallocene catalyst component and a process for preparing such isotactic polyolefins.
Olefins having 3 or more carbon atoms can be polymerised to produce a polymer with an isotactic stereochemical configuration. For example, in the polymerisation of propylene to form polypropylene, the isotactic structure is typically described as having methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer. This can be described using the Fischer projection formula as follows: 
Another way of describing the structure is through the use of NMR spectroscopy. Bovey""s NMR nomenclature for an isotactic pentad is . . . mmmm with each xe2x80x9cmxe2x80x9d representing a xe2x80x9cmesoxe2x80x9d diad or successive methyl groups on the same side in the plane.
In contrast to the isotactic structure, syndiotactic polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Using the Fischer projection formula, the structure of a syndiotactic polymer is described as follows: 
In NMR nomenclature, a syndiotactic pentad is described as . . . rrr . . . in which xe2x80x9crxe2x80x9d represents a xe2x80x9cracemicxe2x80x9d diad with successive methyl groups on alternate sides of the plane.
In contrast to isotactic and syndiotactic polymers, an atactic polymer exhibits no regular order of repeating unit. Unlike syndiotactic or isotactic polymers, an atactic polymer is not crystalline and forms essentially a waxy product.
While it is possible for a catalyst to produce all three types of polymer, it is desirable for a catalyst to produce predominantly an isotactic or syndiotactic polymer with very little atactic polymer. C2-symmetric metallocene catalysts are known in the production of the polyolefins. For example, C2 symmetric bis indenyl type zirconocenes which can produce high molecular weight high melting isotactic polypropylene. The preparation of this metallocene catalyst is costly and time-consuming, however. Most importantly, the final catalyst consists of a mixture of racemic and meso isomers in an often unfavourable ratio. The meso stereoisomer has to be separated to avoid the formation of atactic polypropylene during the polymerisation reaction.
EP-A-0537130 discloses the use of a C1 symmetric metallocene catalysts for the production of isotactic polypropylene. A preferred catalyst is isopropylidene (3-t-butyl-cyclopentadienyl fluorenyl)ZrCl2. This catalyst has a bulky t-butyl group positioned on the cyclopentadienyl ring distal to the isopropylidene bridge. This catalyst has the advantage that it consists of only one stereoisomer and so no isomeric metallocene separation is required at the final stage of its synthesis. Whilst polypropylene preparation using this catalyst produces isotactic polypropylene, the polymer product has poor mechanical properties because of the presence of regiodefects and relatively low molecular weight.
Regiodefects occur in the polymer chain when, instead of producing a perfect isotactic polyolefin in which each monomeric unit is positioned head-to-tail in relation to the next, mis-insertions of the monomers occur so as to give either a head-to-head or tail-to-tail mis-match. In the polymerisation process according to EP-A-0619325, there is a typical mis-insertion frequency of around 0.4%. These so called (2-1) regiodefects are partially transferred to the so called (1-3) insertion through an isomerisation process leaving units of four CH2 groups in the backbone of the polypropylene chain. This has a deleterious effect on the physical and mechanical properties of the polymer and results in low molecular weight isotactic polypropylene with a low melting point.
The present invention aims to overcome the disadvantages of the prior art.
In a first aspect, the present invention provides a metallocene catalyst component for use in preparing isotactic polyolefins, which component has the general formula:
Rxe2x80x3(CpR1R2R3)(Cpxe2x80x2Rxe2x80x2n)MQ2xe2x80x83xe2x80x83(I)
wherein Cp is a substituted cyclopentadienyl ring; Cxe2x80x2p is a substituted or unsubstituted fluorenyl ring; Rxe2x80x3 is a structural bridge imparting stereorigidity to the component; R1 is a substituent on the cyclopentadienyl ring which is distal to the bridge, which distal substituent comprises a bulky group of the formula XR*3 in which X is chosen from Group IVA, and each R* is the same or different and chosen from hydrogen or hydrocarbyl of from 1 to 20 carbon atoms, R2 is a substituent on the cyclopentadienyl ring which is proximal to the bridge and positioned non-vicinal to the distal substituent, and is of the formula YR#3, in which Y is chosen from group IVA, and each R@ is the same or different and chosen from hydrogen or hydrocarbyl of 1 to 7 carbon atoms, R3 is a substituent on the cyclopentadienyl ring which is proximal to the bridge and is a hydrogen atom or is of the formula ZR$3, in which Z is chosen from group IVA, preferably carbon, and each R$ is the same or different and chosen from hydrogen or hydrocarbyl of 1 to 7 carbon atoms, each Rxe2x80x2n is the same or different and is hydrocarbyl having 1 to 20 carbon atoms in which 0xe2x89xa6nxe2x89xa68; M is a Group IVB transition metal or vanadium; and each Q is hydrocarbyl having 1 to 20 carbon atoms or is a halogen. In one embodiment, the metallocene catalyst component of the present invention has the general formula:
Rxe2x80x3(CpR1R2R3)(Cpxe2x80x2Rxe2x80x2n)MQ2xe2x80x83xe2x80x83(II)
wherein Cp is a substituted cyclopentadienyl ring; Cpxe2x80x2 is a substituted or unsubstituted fluorenyl ring; Rxe2x80x3 is a structural bridge imparting stereorigidity to the component; R1 is a substituent on the cyclopentadienyl ring which is distal to the bridge, which distal substituent comprises a group of the formula XR*3; in which X is chosen from Group IVA, preferably carbon or silicon, and each R* is the same or different and chosen from hydrogen or hydrocarbyl of from 1 to 20 carbon atoms, R2 is a substituent on the cyclopentadienyl ring which is proximal to the bridge and positioned non-vicinal to the distal substituent and is of the formula YR#3 in which Y is chosen from group IVA, preferably carbon, and each R# is the same or different and chosen from hydrogen or hydrocarbyl of 1 to 7 carbon each Rxe2x80x2n, is the same or different and is hydrocarbyl having 1 to 20 carbon atoms in which hydrocarbyl having 1 to 20 carbon atoms in which 0xe2x89xa6nxe2x89xa68; is a Group IVB transition metal or vanadium; and each Q is hydrocarbyl having 1 to 20 carbon atoms or is a halogen.
Polyolefins produced using metallocene catalyst component of the present invention are not only found to be isotactic but are also found to be substantially free of regiodefects. Accordingly, the polyolefins produced thereby have improved mechanical properties including a high weight average molecular weight typically of the order of 150,000-600,000 and elevated melting point. Without wishing to be bound by any theory, it is postulated that the bulky group on the cyclopentadienyl ring contributes to the stereospecificity of the polymerisation reaction whereas the proximal substituent(s) on the cyclopentadienyl ring contribute to the regiospecificity of monomer insertion and the increase of molecular weight.
In the bulky distal substituent group R1, X is preferably C or Si. R* may be a hydrocarbyl such as alkyl, aryl, alkenyl, alkylaryl or aryl alkyl, preferably methyl, ethyl, propyl, isopropyl, butyl, isobutyl, amyl, isoamyl, hexyl, heptyl, octyl, nonyl, decyl, cetyl or phenyl. RI may comprise a hydrocarbyl which is attached to a single carbon atom in the cyclopentadienyl ring or may be bonded to two carbon atoms in that ring. Preferably, R1 is C(CH3)3, C(CH3)2Ph, CPh3 or Si(CH3)3, most preferably C(CH3)3.
The proximal substituents R2 and R3 are preferably CH3.
The structural bridge Rxe2x80x3 is preferably alkylidene having 1 to 20 aliphatic or aromatic carbon atoms, a dialkyl germanium or silicon or siloxane, alkyl phosphene or amine bridging the two Cp rings. Rxe2x80x3 is preferably dimethylsilanediyl in which the two Cp rings are bridged by the silicon atom, or isopropylidene in which the two Cp rings are bridged by position 2 of the isopropylidene.
M is preferably zirconium or titanium, most preferably zironium. Q may be a hydrocarbyl such as alkyl, aryl, alkenyl, alkylaryl or aryl alkyl, preferably methyl, ethyl, propyl, isopropyl, butyl, isobutyl, amyl, isoamyl, hexyl, heptyl, octyl, nonyl, decyl, cetyl or phenyl. Q is preferably a halogen.
The fluorenyl ring Cpxe2x80x2 can have up to 8 substituent groups Rxe2x80x2n, each of which is the same or different and may be a hydrocarbyl selected from alkyl, aryl, alkenyl, alkyl aryl or aryl alkyl, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, amyl, isoamyl, hexyl, heptyl, octyl, nonyl, decyl, cetyl or phenyl. These substituents must be selected so that they do not interfere with coordination of the monomer to the metal. Preferably, therefore, the fluorenyl ring is unsubstituted at both positions 4 and 5, these positions being distal to the bridge.
In a further aspect, the present invention provides a metallocene catalyst component for use in preparing isotactic polyolefins, which comprises (i) a catalyst component as defined above; and (ii) a regioisomer thereof in which R2 is proximal to the bridge and positioned vicinal to the distal substituent.
Such regioisomers are frequently relatively easy to prepare because they are formed as a xe2x80x9cby-productxe2x80x9d during the synthetic route by which the catalyst component (i) may be made.
Surprisingly, it has been found that catalyst components including both regioisomers can be used in the preparation of isotactic polyolefins which have a multimodal, especially a bimodal, molecular weight distribution.
In a further aspect, the present invention provides a catalyst system for use in preparing isotactic polyolefins, which comprises (a) a catalyst component as defined above; and (b) an aluminium- or boron-containing co-catalyst capable of activating the catalyst component. Suitable aluminium-containing co-catalysts comprise an alumoxane, an alkyl aluminium and/or a Lewis acid.
The alumoxanes usable in the process of the present invention are well known and preferably comprise oligomeric linear and/or cyclic alkyl alumoxanes represented by the formula: 
for oligomeric, linear alumoxanes and 
for oligomeric, cyclic alumoxane, wherein n is 1-40, preferably 10-20, m is 3-40, preferably 3-20 and R is a C1-C8 alkyl group and preferably methyl. Generally, in the preparation of alumoxanes from, for example, aluminium trimethyl and water, a mixture of linear and cyclic compounds is obtained.
Suitable boron-containing co-catalysts may comprise a triphenylcarbenium boronate such as tetrakis-pentafluorophenyl-borato-triphenylcarbenium as described in EP-A-0427696, or those of the general formula [Lxe2x80x2xe2x80x94H]+[B Ar1 Ar2 X3 X4]xe2x80x94 as described in EP-A-0277004 (page 6, line 30 to page 7, line 7)
The catalyst system may be employed in a solution polymerisation process, which is homogeneous, or a slurry process, which is heterogeneous. In a solution process, typical solvents include hydrocarbons with 4 to 7 carbon atoms such as heptane, toluene or cyclohexane. In a slurry process it is necessary to immobilise the catalyst system on an inert support, particularly a porous solid support such as talc, inorganic oxides and resinous support materials such as polyolefin. Preferably, the support material is an inorganic oxide in its finally divided form.
Suitable inorganic oxide materials which are desirably employed in accordance with this invention include Group 2a, 3a, 4a or 4b metal oxides such as silica, alumina and mixtures thereof. Other inorganic oxides that may be employed either alone or in combination with the silica, or alumina are magnesia, titania, zirconia, and the like. Other suitable support materials, however, can be employed, for example, finely divided functionalised polyolefins such as finely divided polyethylene.
Preferably, the support is a silica having a surface area comprised between 200 and 700 M2/g and a pore volume comprised between 0.5 and 3 ml/g.
The amount of alumoxane and metallocenes usefully employed in the preparation of the solid support catalyst can vary over a wide range. Preferably the aluminium to transition metal mole ratio is in the range between 1:1 and 100:1, preferably in the range 5:1 and 50:1.
The order of addition of the metallocenes and alumoxane to the support material can vary. In accordance with a preferred embodiment of the present invention alumoxane dissolved in a suitable inert hydrocarbon solvent is added to the support material slurried in the same or other suitable hydrocarbon liquid and thereafter a mixture of the metallocene catalyst component is added to the slurry.
Preferred solvents include mineral oils and the various hydrocarbons which are liquid at reaction temperature and which do not react with the individual ingredients. Illustrative examples of the useful solvents include the alkanes such as pentane, iso-pentane, hexane, heptane, octane and nonane; cycloalkanes such as cyclopentane and cyclohexane, and aromatics such as benzene, toluene, ethylbenzene and diethylbenzene.
Preferably the support material is slurried in toluene and the metallocene and alumoxane are dissolved in toluene prior to addition to the support material.
In a further aspect, the present invention provides use of a catalyst component as defined above and a co-catalyst which activates the catalyst component, for the preparation of isotactic polyolefins, preferably polypropylenes. In a still further aspect, the present invention provides use of a metallocene catalyst component comprising (i) the catalytic component and (ii) a regioisomer thereof, in which R2 is proximal to the bridge and positioned vicinal to the distal substituent, for the preparation of isotactic polyolefins, especially polypropylenes, having a multimodal molecular weight distribution, preferably a bimodal molecular weight distribution.
In a further aspect, the present invention provides a process for preparing isotactic polyolefins, especially polypropylenes, which comprises contacting a catalyst system as defined above with at least one olefin, preferably propylene, in a reaction zone under polymerisation conditions.
The catalyst component may be prepared by any suitable method known in the art. Generally, the preparation of the catalyst component comprises forming and isolating bridged dicyclopentadiene, which is then reacted with a halogenated metal to form the bridged metallocene catalyst.
In one embodiment, the process for preparing the bridged metallocene catalyst components comprises contacting a substituted cyclopentadiene having bulky and non bulky substituents on the cyclopentadienyl ring with a fluorene under reaction conditions sufficient to produce a bridged substituted dicyclopentadiene. The process further comprises contacting the bridged substituted dicyclopentadiene with a metal compound of the formula MQk as defined above under reaction conditions sufficient to complex the bridged dicyclopentadiene to produce a bridged metallocene wherein M and Q are each defined as above and 0xe2x89xa6kxe2x89xa64. The process step of contacting the bridged substituted dicyclopentadiene with a metal compound can be performed in a chlorinated solvent.
In a further embodiment, the process comprises contacting a substituted cyclopentadiene having bulky and non bulky substituents on the cyclopentadienyl ring with an alkyl silyl chloride of the formula R{tilde over ( )}2SiHal2 wherein R{tilde over ( )} is a hydrocarbyl having 1 to 20 carbon atoms and Hal is a halogen. A second equivalent of an optionally substituted fluorene is added to produce a silicon bridged substituted cyclopentadienyl fluorenyl ligand. The subs={tilde over ( )}quent steps are similar to those above for producing a bridge disubstituted cyclopentadienyl-fluorenyl ligand coordinated to metals such as Zr, Hf and Ti.
In a further embodiment, the process comprises contacting a substituted cyclopentadiene with a fulvene producing agent such as acetone to produce a substituted fulvene. Subsequently, in a second step, the fulvene is reacted with fluorene to produce a carbon bridged substituted cyclopentadienyl-fluorenyl ligand that will produce the desired metallocene catalysts after reacting with MCl4, in which M is Zr, Hf or Ti.